1. Field of the Invention
The present invention relates to a switch sheet with a dome shaped conductive spring (hereinafter referred to as a “dome shaped spring”) and a switch that utilizes such switch sheet, which is used in electronic equipment such as a portable telephone.
2. Description of Relevant Art
Dome shaped springs are used in pushbutton switches used to turn various kinds of electronic equipment such as a portable telephone, on or off. In order to make contact with a fixed contact occur through operation of a switch, a dome shaped spring must undergo distortion of its shape in response to compression load many times over. Accordingly, thin stainless sheeting is used for this kind of dome shaped spring.
FIG. 1 illustrates an example of the pushbutton switch. There, for the pushbutton switch or switch sheet 101, dome shaped spring 102 is secured to ring shaped fixed contact 106 of printed wired substrate 105, adhered together with plastic sheeting 104 by adhesive agent 103. Dome shaped spring 102 is usually made from thin stainless steel sheeting. When, through actuator 107 located above the center of dome shaped spring 102, spring 102 is clicked via plastic sheeting 104 of switch sheet 101, spring 102 depresses inwards coming into contact with circuit conductor 108, thereby causing conduction between fixed contact 106 and circuit conductor 108. Technology of this kind has been disclosed in Japanese Patent Publication No. 2002-216582.
With this kind of switch, it is preferable for actuator 107 to click the center region of dome shaped spring 102 when this kind of switching operation is performed repeatedly. However, it is very easy for actuator 107 to become positionally misaligned in relation to dome shaped spring 102. The smaller the equipment used, the more pronounced this phenomena becomes. It is therefore necessary to consider solutions appropriate for increasingly miniaturized devices. Due to this problem of misaligned positioning, there is not a smooth tactile response when using electronic equipment (e.g., a portable telephone), and a poor sensation is felt. The clicking of the switch is not perceived distinctly by the fingers. This phenomena called the clicking sensation can be expressed in terms of a click ratio. The click ratio can be expressed as (P1−P2)/P1×100, where (P1) is the load of maximum value a for load—displacement curve, for the dome shaped spring shown in FIG. 3 and (P2) is the load of minimum value b. This expression indicates the clicking sensation. The clicking sensation decreases as the value for the click ratio decreases. A click ratio of 30% or greater is preferable.
Different structures that realize a high click ratio have been investigated. However, to date, nothing satisfactory has been achieved. (See the inventions disclosed in Japanese Patent Publication No. 2001-135189 and Japanese Patent Publication No. 2000-188036 for example.) Furthermore, as mentioned, as electronic devices (e.g., portable telephones) become increasingly small, dome shaped springs also are becoming increasingly miniaturized. Also, actuators are being incorporated with the precision of maximum about 0.5 mm. This makes the problem of a decreasing click sensation through misaligned positioning between a dome shaped spring and actuator still more difficult to resolve. Further, the parts forming a dome shaped spring must be more durable and last longer. Technology that solves these problems is required.
FIGS. 2A and 2B illustrates an enlarged view of examples of dome shaped springs 102. The spring in FIG. 2A is the more standard one used. The spring in FIG. 2B has an upward protrusion 109. A switch including this upward protrusion type is said to be a switch wherein it is comparatively difficult for there to be a deterioration in the clicking sensation when pressed from a point away from center. It is known that where this type of switch is assembled into an electronic device, when the actuator is clicked, if the position of the click is directed from off center of the dome shaped spring, there is no deterioration in the sensation of the clicking operation. However, it frequently happens with a switch including dome shaped spring 102 shown in FIG. 2B, that the upward protrusion 109 is buried in plastic sheeting 104 (FIG. 1) causing the click of dome shaped spring 102 to be out of position.
In the above-mentioned switch, there is a multi-directionally operable switch with directional keys and an execution key. Through one actuator, this multi-directionally operable switch can click in the directions of a cross shape or a central part. Such a multi-directionally operable switch is shown in FIGS. 1, 3, 4 and 5 of Japanese Patent Publication No. 11-331329 in which it is referred to as a multi-function key.
With this multifunction key also, when as described, the clicking operation is performed repeatedly, the problem of misaligned positioning arises causing a poor clicking sensation. Technology that answers the needs of these increasingly miniaturized devices is required but as yet, nothing satisfactory has been devised.